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Is it incorrect to say
I'm 20 years old next month.
I'm learning through an English app. It says that the sentence above is incorrect and the correct answer is
I shall be 20 years old next month.
There were 4 choices to this question in the app:
I (am/shall/shall be/will) 20 years old next month.
I thought present simple is appropriate there since this tense can be used to describe the future.
You certainly can use the present tense (I am, he is, we are, etc) about a scheduled event, and many people do so when discussing a forthcoming birthday. I am sixty tomorrow, I am fifty in March, I am fifty in two years, I am forty in four weeks, I am 35 in a couple of months. You can also say (e.g.) 'I will be 26 in a few weeks', and British users may well use 'shall', which is more formal.
We use the present simple for something scheduled:
We have a lesson next Monday. The train arrives at 6.30 in the morning.
The holidays start next week.
It's my birthday tomorrow.
Talking about the future (British Council)
Wrexham MP Ian Lucas will stand down at next election - 11 Oct 2019 — "I am 60 next year. I have been an MP since I was 40. I think the time is right for me to choose to follow a different path in the years to come" (BBC)
Ex-soldier from Hornchurch in diving world record bid: 14 Feb 2016 — He said: “I am 50 next year and so this is my last chance to get the record. I want to stay underwater for five days." Hornchurch Recorder (Essex newspaper).
On the the more formal side, from the Letters of CH Spurgeon (1834 - 1892), a British preacher and religious writer:
May the everlasting arms be underneath you! I breathe for you a loving, tender prayer,—" Lord, comfort Thy dear servant, and when he departs, may it be across a dried-up river into the land of living fountains!" I am fifty next Thursday, and you are near your Jubilee. In this we are alike; but Jesus is the highest joy. Into the Father's hands I commit you, "until the day break, and the shadows flee away." Your loving brother, C. H. SPURGEON.
Want to add to the accepted answer: using the present tense to speak about future events is common, but to my intuitive understanding as a native US speaker is incorrect... except that it's understood to be a shortening of saying "I am/I'm going to be 20 years old next month."
Saying "I will/I'll be 20 years old next month" is overwhelmingly the most correct/normal way to say it in my experience, but saying "I'm 20 years old next month" is perfectly acceptable, because it's understood to imply the absent "going to be".
Resultingly, it'd seem wrong written down in any context besides the most informal and conversational (personal communication/blog posts/anything emulating face-to-face conversation). In professional communication, essays, or in a book (unless it's in dialogue between characters), use of the present tense would be interpreted as either agrammatical or out-of-place due to its informality.
This is a perfect example of how colloquial speech leaves out or contracts redundant words.
Your third quote shows "I am 20..." as acceptable, and "I'm 20..." is merely the shortened (spoken or even lazy) way of saying the same.
Upshot - your app isn't quite smart enough to know they're the same.
I am is correct it's just a multiple choice answer and one of the choice was I am which the OP chose and the app told him it was wrong, he is the one who shortened it in his question
I am 20 years old next month
I have a feeling that the aforementioned construction derives from a very formal way to express a future event that is considered inevitable or expected, e.g:
Soon I am to be 20 years old.
“We are to be married in the month of June”
The formula be to + infinitive can be replaced by am/is/are going to + infinitive
Google books revealed the following quotations
To-morrow I am to be arraigned as a culprit; my friends may leave me, my counsel may desert my cause as hopeless, and the world may point at me as a recreant to virtue…
To love and To Be Loved 1855
“I am very intimate, especially with Matilda: I am to be one of her bridesmaids. You know, of course, that the wedding-day is fixed for to-morrow?” 1853
“Tomorrow is to be one of our anniversaries, you know,” she replied; “twenty-four years ago – to-morrow — was to have been to me what to-day is to Linnet. I wonder if I were [sic] as light hearted as Linnet.”
“…for science is inexhaustible; every day opens a glimpse of some fact that tomorrow is to see ascertained.” Significance of the Alphabet By Charles Kraitsir, 1846
Today that line might be written as “that tomorrow sees ascertained” or “that tomorrow will see ascertained.”
The TL;DR version:
"I am 20 years old next month" is incorrect because in that sentence "20 years old" is an adjective which describes you.
Remember that the verb 'to be' plus an adjective is used to talk about the present.
"It is my 20th birthday next month" is correct because the object of the sentence is the birthday, and is a scheduled event in the future.
Original answer follows:
I'm 20 years old next month.
Is understandable but grammatically incorrect, although frequently used colloquially.
Begin edit
As an aside. @Muzer comments that "It's not grammatically incorrect. -1.".
It is incorrect in that form, because if you want to use "I am" to speak about the future then it should be combined with a verb ending in "-ing". Here's how you can use "I am" correctly:
I am going to be 20 next month.
I'm turning 20 next month.
If you're talking about the birthday, as a scheduled event, then you can use the present tense like this:
It is my 20th birthday next month.
For more, see https://learnenglish.britishcouncil.org/english-grammar-reference/talking-about-the-future
End edit
I'll be 20 years old next month.
Is grammatically correct, but in my opinion:
I'll turn 20 next month.
Is the best way of saying what you want to say.
If you're currently 20 then it's not incorrect to say you'll be 20 next month, too; but to say that you will turn 20 suggests that your 20th birthday is next month.
